Anti-mine protective coating for ships and marine structures

ABSTRACT

A protective coating applied to the underside of vessels, ships or ferrous marine structures prevents explosives or limpet-type mines that attach via magnets from attaching below the waterline. The coating provides shielding and attenuates the field strength of the magnetic flux created by magnets that hold the explosives or mines in place.

BACKGROUND OF THE INVENTION

Mine warfare includes acts of laying floating or submerged explosive devices in harbors, shipping lanes, or strategic choke points to preserve or deny the use of the seas. Mine warfare also includes placing explosive mines on the hulls of shipping vessels to sink or damage the vessel.

There are several different types of mines used in mine warfare. These types of mines include contact mines, magnetic influence mines, acoustic mines, pressure mines and limpet mines. Contact mines explode when a ship strikes a spine extending from the mine. Acoustic and pressure mines detonate when a particular sound signature or a pressure wave from a ship passing above the mine occurs, respectively.

Magnetic influence mines comprise detonator circuitry that detects an electric or magnetic field of a ship passing over the mine. Typically, these mines are equipped with a search coil or magnetometer that senses a change in the earth's magnetic field caused by the presence of a moving ship. The detonator circuit analyzes magnetic inputs through a preprogrammed firing circuitry such that when the magnetic field reaches a preset threshold, the detonator circuitry explodes the mine.

Limpet mines are explosive charges that are used to disable or destroy vessels and ships during war. In use, a limpet mine is attached below the water line of a vessel near the engine and detonated via a timed fuse or remote control. The blast created from the detonation rips an opening into the ship sinking or disabling it. A limpet mine typically comprises a PVC outer shell filled with plastic explosives and having a base plate. The base plate includes magnets that attach the mine to a ferrous surface. Since the magnets that secure the limpet mine to the hull of the target vessel are small in size and weak in magnetic field strength, they can be defeated by providing a sufficiently thick layer of non-ferrous material on the exterior surface of the hull of the vessel.

Most modern limpet mines incorporate an anti-removal triggering device that increases the probability of a detonation once planted. Typically, a limpet mine remains undiscovered until after detonation of the mine. This occurs because the mines are easy to deploy against targets. To counter the threat of limpet mines, navies currently put divers in the water or remotely operated vehicles to inspect the hulls of vessels. Even if a mine is located after it has been planted, the likelihood of successful removal is minimal, since removing and disarming it is unreliable. These mine removal techniques are time consuming, very costly procedures to implement and extremely dangerous to the divers.

Since, magnets secure limpet mines to metal structures, if the strength of the magnetic fields produced by these magnets is reduced, the magnets may fail to properly secure the limpet mine to a metal structure. A magnetic field results from a source of magnetic flux, which is a term of art that refers to the total amount of magnetic field. Sources of magnetic fields include the earth, magnets, motors, and large ferrous objects such as vessels or ships. Magnetic fields are measured in units of gauss and oersted. Magnetic flux density is measured in gauss, whereas magnetic field strength is measured in oersteds. The ratio of magnetic flux in gauss to magnetic field in oersteds in a material is defined as permeability.

Permeability is a measure of the properties of a material to absorb a magnetic field. The permeability of ferromagnetic and ferromagnetic materials is high; whereas the permeability of air is one. No known material totally blocks magnetic fields. However, some materials may attenuate or lessen the amount, force, magnitude, or value of the magnetic field. These materials cause the lines of flux to move farther apart, resulting in a decrease in magnetic flux density compared with a vacuum, are called diamagnetic.

Thus, the present invention is directed towards a coating that attenuates a magnetic field created by a magnet that secures a limpet mine to a ferrous surface.

Since an underwater portion of a hull is subjected to the corrosive salt water environment and under constant attack by biological marine growth, many modern ships, such as those used in the U.S. Navy, include an exterior, underwater coating of an anti-corrosive primer covering top coated with an anti-fouling coating. Fouling occurs when biologicals such as barnacles, tubeworms and sea grasses attach to a ship's hull and continue to grow there. A fouled hull can reduce a ship's speed by as much as 5 percent and increase fuel consumption by 40 percent.

Typically, the anti-fouling coating comprises copper and extends from the bow of the vessel to the stern. Since copper exhibits high magnetic properties, coating a ship's hull with an anti-fouling coating, as mentioned above, may increase the magnetic signature of the ship. Moreover, the copper is a toxicant that leaches into the ocean and represents an environmental hazard that is subject to ever-increasing regulations. Thus, there is a need for a coating that can control growth of biologicals on submersed surfaces of vessels.

Polyurethane coatings have excellent hardness, abrasion and chemical resistance and are very durable. When these coatings are applied to a ferrous surface, they provide attenuation of magnetic flux thereby reducing the total magnetic field and flux linkage between a magnet and the ferrous surface. If applied in sufficient thicknesses, the coatings can prevent magnets from attaching to the ferrous surface. Moreover, when the coating is impregnated with a biocide, then it also exhibits antifouling properties. The biocides used may include one or more of the following: calcium oxide, cerium oxide, magnesium oxide, titanium oxide, zinc oxide, aluminum oxide, copper oxide, tributyltin, diuron, copper thiocyanate, Irgarol 1051, zinc pyrithione and dichlofluanid.

Other types of materials that may be used as an alternative coating may include one-component and two-component polyurethanes including aliphatic and aromatic polyurethanes, acrylic epoxy, acrylic/urethane blends, nylon, polyester, polyvinyl chloride, vinyl plastisols or any combination thereof. Aliphatic polyurethanes are based on aliphatic isocyanates (HDI and IPDI) and polyester and/or acrylic polyols; whereas aromatic polyurethanes are based on aromatic isocyanates (MDI and TDI) and polyether polyols. It should be noted that any coating with a high permeability might be used to coat the submersed surface.

The present invention is a counter-measure for magnetic influence and limpet mines and includes a coating that reduces the magnetic influence from a ship. The coating prevents magnets that are typically used to secure limpet mines and to hulls of ships and ferrous support structures, from attaching to the ships or structures. Thus, the explosives cannot be seated properly using magnetic fastening means. An added benefit of the present invention is the reduction of electrolysis created when the metal hull of a vessel contacts salt water. Since, the hull is coated below the water line, it is not exposed to the deteriorative effects of electrolysis. Additionally, the potential of a reduced acoustic and magnetic signature is highly probable thereby lessening the threat of triggering a bottom mine.

BRIEF SUMMARY OF THE INVENTION

Generally, the present invention relates to an anti-magnetic exterior coating for a ferrous surface that prevents explosives secured with magnets from attaching to the ferrous surface. More specifically, the invention relates to polyurethane or other non-ferrous coating and having a low permeability that reduces the magnetic flux that links the magnets to the ferrous surface. Thus, the total magnetic field is reduced, thereby preventing the magnets from being attached to the ferrous surface.

It is an object of the invention to provide an exterior coating for preventing limpet type mines from being deployed against vessels coated with the anti-magnetic coating of the present invention.

It is a further object of the invention to provide a coating for ferrous support members of a marine structure that prevents magnets from being attached thereto.

It is an object of the invention to provide a method for protecting sea-going vessels from having limpet type mines from being attached thereto.

It is an object of the invention to provide a process for applying the anti-magnetic coating to ferrous surfaces.

It is another object of the invention to provide a coating having a high permeability and being impregnated with biocides. This coating replaces both the anti-fouling and anti-corrosive coatings currently used on hulls of vessels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom plan view of a prior art limpet type mine.

FIG. 2 is an elevation view of a prior art mine of FIG. 1.

FIG. 3 is a cross-sectional view of the coating and underlying substrate.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a limpet-type mine 10 that may be planted on a vessel or support member of a marine structure. The mine 10 is hemispherical in shape and includes a flat base-plate 12 equipped with four magnets 14. Magnetic attraction holds the mine in place for deployment. A PVC outer casing 16 encloses an electronics chamber for housing the arming and detonation circuitry as well as the explosive materials. The mine 10 is equipped with an anti-lift plunger 18 that prevents the mine 10 from being removed after being attached. An arming pin 20 extends through a sidewall 22 of the casing 16 and through the anti-lift plunger 18.

FIG. 3 is a cross-sectional elevation view of the coating 1 shown deposited onto a ferrous magnetic metal surface 2. In the preferred embodiment, polyurethane is deposited onto the metal surface in a thickness no less than 0.0625″. However, it should be noted that this thickness may vary according to the permeability of the coating material selected from the group of materials listed above.

The process for applying the anti-mine coating is now discussed. The metal surface onto which the coating is applied is cleaned, preferably by stripping off all loose materials and coatings. It is important that the surface must be free of contaminants. The stripping process may include known methods readily recognizable by those of ordinary skill in the art and may include sand blasting, pressure washing etc. The metal surface is allowed to dry. Depending upon the type of coating material to be used, a primer coating is applied to the metal surface. The anti-mine coating is then applied to the metal surface and allowed to dry. Ships and vessels may be placed into dry dock during the application process.

However, structural support members for marine structures may have the coating applied while in a marine environment. In this instance, a waterproof membrane, such as a sheet of flexible plastic, is place around the support member. Water is evacuated from within the membrane. The metal surface of the structural support is stripped as mentioned above and allowed to dry. A primer is applied to the metal surface if necessary and the anti-mine coating is applied and allowed to dry before the membrane is removed. 

1. An anti-magnetic coating for a vessel having a hull, said anti-magnetic coating covering at least a portion of said hull that extends below a water line and having diamagnetic properties.
 2. The anti-magnetic coating of claim 1 consisting of one or more selected from the following: aliphatic polyurethanes, aromatic polyurethanes, acrylic epoxy, acrylic/urethane blends, nylon, polyester, polyvinyl chloride, and vinyl plastisols.
 3. The anti-magnetic coating for a vessel of claim 2 further including one or more of a biocide selected from the group consisting of: calcium oxide, cerium oxide, magnesium oxide, titanium oxide, zinc oxide, aluminum oxide, copper oxide, tributyltin, diuron, copper thiocyanate, Irgarol 1051, zinc pyrithione and dichlofluanid.
 4. The anti-magnetic coating for a vessel of claim 1 further including one or more of a biocide selected from the group consisting of: calcium oxide, cerium oxide, magnesium oxide, titanium oxide, zinc oxide, aluminum oxide, copper oxide, tributyltin, diuron, copper thiocyanate, Irgarol 1051, zinc pyrithione and dichlofluanid.
 5. The anti-magnetic coating of claim 1 having a thickness of not less than 0.0625″.
 6. A vessel comprising: a propulsion means for driving said vessel through water; a metal hull surrounding at least a portion of said propulsion means; and, an exterior coating on said metal hull consisting of one or more selected from the following: aliphatic polyurethanes, aromatic polyurethanes, acrylic epoxy, acrylic/urethane blends, nylon, polyester, polyvinyl chloride, and vinyl plastisols
 7. The vessel of claim 6 further comprising: one or more of a biocide selected from the group consisting of: calcium oxide, cerium oxide, magnesium oxide, titanium oxide, zinc oxide, aluminum oxide, copper oxide, tributyltin, diuron, copper thiocyanate, Irgarol 1051, zinc pyrithione and dichlofluanid.
 8. A ferrous magnetic support member for a marine structure comprising: a ferrous magnetic material; and a coating on an exterior surface of said ferrous magnetic material, said coating consisting of one or more selected from the following: aliphatic polyurethanes, aromatic polyurethanes, acrylic epoxy, acrylic/urethane blends, nylon, polyester, polyvinyl chloride, and vinyl plastisols.
 9. The ferrous magnetic support member of claim 8 further including: including one or more of a biocide selected from the group consisting of: calcium oxide, cerium oxide, magnesium oxide, titanium oxide, zinc oxide, aluminum oxide, copper oxide, tributyltin, diuron, copper thiocyanate, Irgarol 1051, zinc pyrithione and dichlofluanid. 